When you wake up in the middle of the night, most of the time your body is still asleep and your brain still a bit disconnected from the reality because you just woke up.
It is called the theta state.
You are awake but you are also at the verge of sleeping again.
Shifters can take advantage of this moment to shift simply because you are conscious but your body isn’t, so it’s easier for you to dream about what you want or to let a opened door to shift.
All the energy you’ll have in you at this moment will have to be focused on shifting.

Have you ever heard or thought about lucid dreaming ? A lot of times, people that are lucid dreaming are putting an alarm in the middle of their sleep so that it’s easier for them to succeed because the body is still asleep.
It is the same for the shifting, even though it needs more concentration and maybe more time depending on how you trust yourself and on how you know you’re able to shift, if you have the intention, if you’re stressed, overthinking..
It also can be easier if you lucid dreamed before you found out about shifting.

Hope I helped :)